In an image recording apparatus of this type, a beam emitted from a light source, such as a laser light source, is modulated in accordance with a recording signal and is incident on a rotational polygon mirror, is reflected by a reflecting surface of the polygon mirror to perform horizontal scanning (main scanning), and is incident on a recording medium to perform recording.
FIG. 1 is a schematic diagram of a recording optical system of a conventional image recording apparatus. The apparatus has a laser light source 1, beam expanders 2 and 3, mirrors 4, 5, and 6, an acoustic wave optical modulator (to be referred to as an AOM hereinafter) 7, cylindrical lenses 8 and 9 for outputting beams having substantially circular beam spots, a rotational polygon mirror 10, an f.theta. lens 11 as a focusing means, and a recording medium 12 such as a photosensitive member.
When a halftone image is recorded by a gray scale method with the above image recording apparatus, the digital data of each pixel of an original is converted into an analog signal by a D/A converter, the light beam is intensity-modulated (light amount control) by the analog signal, and recording is performed, thereby reproducing the density of the original. This method can be adopted when a relationship between an energy supplied to the recording medium and the recording density (H-D characteristics) is comparatively linear. As a recording medium providing linear H-D characteristics, a photosensitive material such as a film or print paper is currently available. In an ink-jet printer or a thermal printer, recording is performed mainly in accordance with an area change method.
In recording using the conventional gray scale method, modulation levels equal in number to gray levels must be used. For this reason, since the number of gray levels is determined by the number of bits of a D/A converter, a D/A converter having a large number of bits is needed in order to obtain a large number of gray levels.
In the rotational polygon mirror as a scanning means used in the image recording apparatus of this type, an error in the parallel degree of each reflecting surface with respect to its rotating axis, i.e., an inclination angle error exists. This error causes an error in the main scanning pitch.
If a high-precise rotational polygon mirror having substantially no error is used, such a problem is solved. In this case, however, a very high performance is required for the rotational polygon mirror. In binary image recording, if an allowable pitch error is suppressed to 20 .mu.m, the inclination angle error is about 7 seconds when f=300 mm, which are the practical limitations when considering the manufacturing cost. In halftone image recording, the pitch error must be suppressed below 1 .mu.m. In this case, the inclination angle error is below 0.5 second, which is, technically, almost impossible to realize.
In another method, a galvanometer mirror is used as a scanning means. In this case, scanning is performed by reciprocally rotating a single mirror. Although an inclination angle error does not occur due to its operation principle in this case, the scanning speed is comparatively lower than the rotational polygon mirror, and the scanning angle cannot be widened as compared to the rotational polygon mirror. Since a considerable jitter is generated, an encoder must be used in some cases. In addition, the galvanometer has an error called a wobble, which causes an adverse effect in halftone image recording.
In order to solve the above problems, an elongated cylindrical lens 13 is conventionally arranged in the main scanning direction between the f .theta. lens 11 as the focusing means and the recording medium (photosensitive material in FIG. 1) 12, as shown in FIG. 1, in order to correct the inclination angle error. The cylindrical lenses 8 and 9 are also arranged on the input optical path of the rotational polygon mirror 10 in order to shape the beam to have a substantially circular beam spot on the recording medium 12.
However, the correction method with the arrangement shown in FIG. 1 requires an expensive correcting optical system. The elongated cylindrical lens is especially expensive. In addition, the image surface corrected by this method is curved (as indicated by a broken line) as shown in FIG. 2. A complete inclination angle correction for the entire main scanning surface cannot thus be performed. The curved degree of the image surface is normally about 3 to 4 mm, although it varies according to each optical system. A gap X is thus formed between the image surface and an image-forming plane 15. In halftone image recording, even a curvature of this degree is visually recognized as a pitch error resulting from an inclination angle error. Furthermore, in this correcting method, the correcting cylindrical lens 13 is often arranged comparatively in the vicinity of the image-forming plane 15, and the beam is thus incident on it with a small spot size. Therefore, dust or the like on the lens 13 tends to influence the recording quality and thus leads to a degradation in image quality. This influence is enhanced when a plastic lens is used for cost savings. Regarding adjustment of the cylindrical lens arrangement, it is difficult to perform rotational adjustment of the cylindrical lenses 8 and 9 in the beam incident side of the rotational polygon mirror about their optical axes. It is also difficult to perform adjustment of the cylindrical lens 13 in the beam output side of the rotational polygon mirror along its optical axis.